In the computer industry, there is a constant demand for products that are both less expensive and have higher performance. The hard disk drive used in virtually every personal computer has traditionally been one of the most expensive components installed therein. Thus, disk drive manufacturers have continuously tried to decrease the cost of their drives while at the same time increase the amount of mass storage provided.
As is well known in the art, any rotating mass storage device includes at least one rotating disk, a read/write transducer deposited upon a slider structure, a head positioning assembly and drive electronics. The drive electronics typically include a preamplifier chip and read channel circuitry carried on a circuit board attached to the head/disk assembly. In the past, very small diameter twisted solid wires have typically been used to interconnect the read/write head with the drive electronics. However, there has been a trend in the disk drive industry to integrate the wires with a flexure structure, for improved performance and ease of assembly. Examples of such a configuration is disclosed in U.S. Pat. No. 5,006,946 to Matsuzki and U.S. Pat. No. 5,491,597 to Bennin et al. Such structures typically employ stainless steel flexures having deposited insulating and conductive trace layers and connection pad arrays for electrical interconnection. Each bonding pad of the connection array is typically solder coated.
A representative bonding pad 12 of a typical interconnect 11 is shown in FIG. 1a. An adjacent solder covered bonding pad 14, representing that of a disk drive electronic component 15, is shown. One drawback of this type of solder covered bonding pad occurs during the soldering process, and is illustrated in FIG. 1b. As the solder 13 is liquefied, and the two bonding pads are brought together, the weight of interconnect 11 forces the molten solder out from between the bonding pads due to the lack of a physical gap therein between. The ejected solder creates the detrimental possibility of bridging adjacent joints.
One alternative, which improves upon the solder covered planar bonding pads of FIG. 1a, is a ball-shaped solder bump 22, shown in FIG. 2a. Solder bumps are typically made of conductive metals such as nickel or copper and, as shown in FIG. 2a, covered with solder 23. The solder bump 22 is typically fabricated by using a process known as "electroplating", to build up the conductive material, thereby creating a bump. The bump is then covered with solder. Solder bumps are an improvement over planar bonding pads because, as shown in FIG. 2b, the bump creates a localized high point area so that during the soldering process, a clamping pressure (not shown) can be applied to ensure that the soldered area will contact the adjoining circuit structure 24. In addition, the mechanical portion of the solder bump 22 creates a single point mechanical stop so that when the solder liquefies, the circuits will not come together without a gap therein between. Typically, solder is liquefied or reflowed, by known methods such as thermal conduction or infra-red (IR) heating. As shown in FIG. 2b, after reflow, the bump 22 leaves a gap 28, enabling the solidified solder 13 to form a fillet around the solder bump 22. However, the disadvantage of solder bumps 22 is that its fabrication in the flexure/conductor requires additional manufacturing steps, making the interconnects more expensive. In addition, during processing, contamination in the area where the bump is electroplated can cause weak joints and potential reliability problems. Resulting fractures at the electrical trace and solder bump interface have also been detected, creating reliability concerns.
Other alternative approaches of joining the flexure/conductor structure with drive electronic circuitry include ultrasonically bonding the two circuits and gold ball or aluminum wedge wire bonding. Neither of these approaches are manufacturably desirable because neither approach is reworkable.
Thus, a hitherto unsolved need has remained for a low cost and reliable solder bump which is reworkable.